1. Field of the Invention
This invention broadly relates to liquid crystal matrix panels and more particularly it refers to a control method for matrix panels of a direct addressing, ferroelectric liquid crystal (FLC) type, to enable their improved operation.
2. Description for the Related Art
As it is known, the panels to which this invention relates are used in devices for displaying images and for optical computation applications, both of the projection and of the direct vision types. In these devices, each picture element (pixel) ideally corresponds to the intersection of an element of a first electrode set (for instance arranged as rows) and an element of a second electrode set (for instance arranged as columns) and materially it corresponds to an electro-optical cell comprising a ferroelectric liquid crystal in the room existing between two facing electrodes belonging to the above mentioned two electrode sets. In usual arrangements, a pair of crossed polarisers operatively completes the cell and makes visible the orientation changes of the director in the liquid crystal that can be of the smectic C chiral type.
The panel consisting of FLC cells can be electrically controlled according to various addressing modes (or schemes) or modes for applying voltages and currents to the two electrode assemblies, so as to determine the states of all cells, the number of which is usually much higher than the number of electrodes. The main object of this invention is to provide a novel addressing method as hereinafter disclosed.
The device as a whole comprises the assembly of the described panel with the related electronic circuitry to generate the various voltage signals needed for its operation and the interconnection elements to the panel electrodes. According to the expected application, in addition, polarisers, colour filters, light sources and an optical system can be provided therein.
This invention additionally consists in the device comprising the above set forth assembly and operating according to the hereinafter described control method.
More precisely, this invention relates to a directly addressed FLC matrix panel wherein the ferroelectric liquid crystal cells operate according to a bistable or multistable behaviour in the absence of voltage or in the presence of a continuously applied, high frequency voltage having a sufficient and suitable rms amplitude, known as a high frequency or alternated current stabilisation voltage. As it will be explained, such a role can be played by the control voltages used, in particular, by the data voltages.
Under the term high frequency stabilisation, the phenomenon is usually meant where the stable states of a cell, when a high frequency voltage is present, are closer to the states that can be achieved by the continuous application of a dc voltage. A broader meaning is allotted in this patent description to the above term, since it also includes the phenomenon according to which the relaxation of a cell to a stable state becomes faster when a high frequency voltage is present.
The ferroelectric liquid crystal can be of the smectic C chiral type and the cells can be of the chevron type or of the partially or totally straightened up chevron type. In both cases, the smectic layers are approximately broken up into two halves, which are tilted in opposite directions with respect to a line normal to the cells, at an angle almost equal to (between 110% and 75% in the first case) or much smaller than (between 0% and 75% in the latter case) the characteristic angle of the SmC phase. Multi-stable behaviours can be related to microdomain mixtures of a number of stable states and the crystal can be utilised for storage of intermediate shades. Reference is made, for instance to P. Maltese, "Advances and problems in the development of ferroelectric liquid crystal displays", in Molecular Crystals and Liquid Crystals, Gordon and Breach, vol. 215, pages 57 and following and to the references cited therein.
By means of spaced apart rectangular pulses, of alternately opposite polarities, it is possible to obtain, as a result of each pulse, a cyclic transition of a cell from one extreme state to the other, possibly when a high frequency stabilisation voltage Vhf having a predetermined rms amplitude is present between such pulses. This effect occurs when such pulses have a duration which is higher than a sufficient value, that is a function of the amplitude of the pulses themselves (for a given rms stabilisation voltage). Such sufficient duration has a minimum value, corresponding to a voltage Vtmin, below which the product of each sufficient duration by the corresponding pulse voltage varies to a small extent but at the same time it has a minimum value Amin in the voltage range between one and eight tenths of Vtmin. Often it is not possible to apply to the cells--without damaging them--voltages sufficiently high to observe that the sufficient duration of the pulses increases as the voltage increases: in such case, the Vtmin should be evaluated by extrapolating the behaviours of the cells as observed at the applicable voltages and Amin will be the minimum value of the product duration-voltage in the range of the applicable voltage of one to eight tenths of Vtmin, or the value of the maximum applicable voltage, when it is less than one tenth of Vtmin.
A uniform cell is characterized by the above said three parameters, among which Amin is the most important, as well as by the dependence of Vtmin and Amin on Vhf. In view of the operation of the cell in the high voltage addressing modes, also comprising the subject-matter of this invention, as it will be further described, the significant values of Vtmin and Amin shall be determined in correspondence to a rms amplitude of Vhf equal to the one resulting from the addressing voltages used and, more precisely, from the data voltages and from any stabilisation voltage. As a matter of fact, such parameter values change from cell to cell of the panel, due to manufacturing tolerances (such as thickness differences) or to operation tolerances (such as temperature differences).
A mathematical model describing the operation of the cell during addressing is reported by P. Maltese et al in Digest of Technical Papers of 1993 Intl. SID Symposium, page 642 and following, available by Society for Information Display, 1526 Brookhollow Drive, Suite 82, Santa Ana, Calif. 92705-5421, as well as by P. Maltese et al, in vol. 15 (1993) of "Liquid Crystals", page 819 and following, as well as in the references cited in said scientific papers. Sufficiently small values of Vhf and Vtmin are achieved, as desired, when a large enough positive biaxiality is available of the dielectric constant tensor of the liquid crystal. For its definition, reference is made to the description of the model in the above quoted papers.
Many known addressing modes for FLC panels contemplate different operations wherein, by means of said voltage signals, it is possible to store all changes with respect to the previous image or to store a new image (write), after having erased the previous image (erasure or blanking), in well defined time intervals, which is meant by the "refresh" of the panel. Between successive refreshes, it is possible to hold images stored on the panel, both when voltages are absent and when voltages are present to control other portions of the panel and when any high frequency stabilisation voltages are present. As a matter of fact, the refresh rates are suitable to display also moving images.
In many cases, the display refresh is carried out electrode by electrode of a first set, according to a scanning scheme wherein the writing operation is contemporaneously performed for all pixels belonging to a given electrode, for instance row by row. This very common case, namely a row-by-row scanning scheme, will be often referred to hereinafter, by way of exemplification and not by way of limitation, for the sake of concreteness and simplicity of explanation. It should be apparent, in fact, that the roles of the rows and of the columns can be exchanged and that the electrodes can be arranged according to a quite different geometrical pattern.
Many already known addressing methods, therefore, provide for refreshing the panel on the basis of successive rows, in usually partially overlapping times, as determined by scanning or selection voltages applied to the row electrodes, independent of the images to be displayed. Said selection voltages, in correspondence to the refreshes, can comprise in the first place one or more pulses, namely even variable voltages, of substantially the same polarity in a finished time span, effecting blanking. These cause the erasure of the previously stored image, i.e. they switch the cells of a row into a well defined state, independently of the concurrently applied column voltages. As is also known, such erasure can also be carried out concurrently to the erasure or writing of other rows. The selection voltages corresponding to the refreshes additionally comprise one or more subsequent pulses causing the cells of the concerned row to be switched from an initial state into a final state depending on the voltages, in turn depending on the images to be displayed, applied to the columns, within a single time window, designated as a control window in the present specification. As is known, in the absence of erasure, during a write operation, it is possible to control the state changes in only one direction and, in the refresh cycle, it is necessary to repeat the write operation with signals of opposite polarity in the selection voltages.
Among the above said subsequent pulses corresponding to a write operation, almost always in the prior art a last pulse exists to which a transition between extreme states can correspond, depending on the data voltages existing in the control windows. Such a pulse is designated in this specification as a write pulse. It can be preceded by polarisation pulses and can be followed by stop pulses, as described in the scientific papers published by this inventor, to which direct or indirect reference has been made. Furthermore, it can be preceded by pulses aimed at compensating the effects of any manufacturing differences and of the temperature changes among the cells of the panel, as also described in Italian Patent Application RM93A000567 and in the paper by P. Maltese, on pages 371 and following of the proceedings of 13th International Display Research Conference (1993), available from the Society for Information Display.
The control window can be shorter than the comprehensive duration of all said subsequent pulses. The minimum time difference between selection voltages that can be employed in respect of two different rows is designated as the row (or line) addressing time and it determines the number of rows that can be addressed between two refreshes. Usually, it is the same as the total width of the control window, thereby avoiding undesired content overlapping between successive control windows. The selection time, on the other hand, is the time lapsing from the beginning of a first pulse and the end of the last pulse in the selection voltage, in respect of a selection operation. It should be small in comparison to the time interval between two successive refreshes, even if, on the other hand, it can be large with respect to the row addressing time.
At each refresh, therefore, the display control procedure provides for controlling the rows one by one in successive time windows. In one time window as defined by a selection voltage, the latching is controlled, in all of the cells in the corresponding row, depending on the previous states and on the data voltages applied to the column electrodes in the time window, as functions of the image to be modified.
In any case, selection voltages are applied to the electrodes of a first set and each of these voltages is associated, at each refresh of the display, to a different control time window for all of the cells corresponding to the electrode of the first set (the selected electrode). To the electrodes belonging to the second set data voltages are applied, each of which is formed by superposing the data voltage segments, applied within the different time windows associated to the selection voltages, the segments being designed for controlling all of the cells corresponding to the electrode belonging to the second set. Each pixel of the image to be displayed determines, in the case of a complete erasure of the previous image, the data voltage pertaining to the electrode of the second set within the time window corresponding to the electrode of the first set. In a general case, said data voltage can also depend on the previous images on the same pixel as well as on correction factors connected to the preceding and following data voltages.
It is known that, to avoid undesired effects of state changes of cells not belonging to the selected electrode, each data voltage segment must have the same average value (as computed in each corresponding window), independent of the corresponding cell and of the state it should take. In addition, each data voltage and each selection voltage must have identical average values (for the complete waveform), independent of the data assembly (of the image) and of the concerned electrode. Without jeopardising the broad concepts heretofore set forth, the above mentioned average value will be considered in the following description as a reference value with respect to which each voltage will be measured and it is therefore specified to be null.
All above described features are common to both the addressing method according to this invention and to the prior art addressing methods.
In the above already mentioned works of P. Maltese et al., systems and methods as well as a mathematical model are disclosed for controlling a matrix panel in which each picture element (pixel) ideally corresponds to the intersection of an element of a first electrode set and an element of a second electrode set and materially it corresponds to an electro-optical cell comprising a ferroelectric liquid crystal in the room existing between two facing electrodes belonging to said two electrode sets, said multistable cell featuring a minimum product pulse time.times.pulse voltage Amin, within the voltage range from one to eight tenths of the pulse voltage allowing the minimum pulse time Vtmin, when spaced apart rectangular pulses having alternately opposite polarities are applied switching the cell from an extreme state to the other one, when between the pulses a high frequency voltage of constant rms amplitude Vhf is applied, and in which selection voltages are applied to the electrodes of the first set and each of these voltages is associated, at each selection operation, to a different control time window, namely a time window allocated to the control by the voltages of the stable states by all the cells corresponding to the electrode of the first set, and in which data voltages are applied to the electrodes belonging to the second set and each of these data voltages is formed by superposing the data voltage segments for each pixel, namely the voltages applied within the different control time windows associated to the selection voltages and designed to control each of the cells corresponding to the electrode belonging to the second set, said voltage segments depending on each data item describing a pixel of the image to be displayed in correspondence to the electrode of the second set, in description pixel-by-pixel of the image, and wherein, upon extracting the possible voltage function of the time added to all said voltages to facilitate the implementation of the circuits generating them, each data voltage has an identical average value independent of the considered electrode, of the position of the pixel and of the data item, and each selection voltage has an identical average value as the data voltages, taken as a reference value for measuring each voltage.
IBM Technical Disclosure Bulletin, Vol 36, No. 1, January 1993, New York, U.S., pages 446-447, discloses a three slot drive scheme for multiplexing a ferroelectric liquid crystal shutter device, namely a method for controlling a ferroelectric liquid crystal panel. The scheme consists of on- and off-waveform for the column data lines and select and non-select for the row select lines. The on- and off-data have exactly opposite phases with voltage amplitudes varying between positive and negative polarities with a zero voltage period inbetween, such that a net zero DC is always maintained. The non-select waveform is a constant zero voltage. The select waveform consists of non-equal amplitude voltages V1, V2, V3, with V1 and V2 having the same polarity and V3 having opposite polarity.
It is an object of this invention to provide an addressing method overcomes the limitations of the fastest methods of the prior art and in particular to obtain shorter row addressing times or an extended range of operation conditions.
All or almost all addressing methods of the prior art provide for using an uninterrupted write pulse in the selection voltages. In slower methods, capable of operating also at low voltage, the control window completely contains the write pulse. The fastest methods of the prior art, just as the method of this invention, can utilise a control window shorter than the write pulse and are based upon the use of relatively high voltages comparable to Vtmin, so that the tensor dielectric properties of the FLC become important. Reference is particularly made to the "fast" and "superfast" methods, as described in the publications of the inventor and to the modes based upon unipolar pulses, as developed during the British project JOERS/Alvey, in connection with which reference can be made to an article of D. G. Mc Donnel et al, on page 654 and following of the above quoted 1993 SID Digest, as well as to the paper of J. R. Hughes and E. P. Raynes, on page 597 and followings, of vol. 13 (1993), with errata corrige on page 281 and following of volume 15 (1993), "Liquid Crystals", a scientific journal, Taylor and Francis, Great Britain, as well as in the references cited in said scientific papers. The addressing modes of both classes allow use of a control window shorter than the write pulse, overlapping the end of the write pulse and the beginning of a stop pulse, in the case of the "fast" and "superfast" modes, and overlapping the beginning of the write pulse, in the case of modes based upon unipolar pulses.
As it appears from the above mentioned references, preliminary to this invention the matrix addressing problems of FLC cells have been closely investigated and various novel addressing modes have been proposed and, more recently, a simplified model of a ferroelectric liquid crystal cell has been achieved, as well described in the above mentioned publications of P. Maltese et al. The model takes into account the tensorial dielectric properties of the material and it is capable of forecasting the operation of the cell in matrix addressing conditions.